diff -u b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
--- b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
@@ -363,9 +363,7 @@
     // Disallow references to the referencing entity.
     $entity = $this->configuration['entity'] ? $this->configuration['entity'] : NULL;
     if ($entity instanceof EntityInterface) {
-      // To maintain backwards compatibility: if the setting is not set, then
-      // allow referencing self.
-      $allow_self_reference = isset($configuration['handler_settings']['allow_self_reference']) ? !empty($configuration['handler_settings']['allow_self_reference']) : TRUE;
+      $allow_self_reference = !empty($configuration['handler_settings']['allow_self_reference']);
       if (!$allow_self_reference) {
         // Referencing entity must be the same entity type as the query.
         if ($entity->getEntityTypeId() == $entity_type->id()) {
diff -u b/core/modules/system/tests/src/Kernel/Entity/EntityReferenceSelectionReferenceableTest.php b/core/modules/system/tests/src/Kernel/Entity/EntityReferenceSelectionReferenceableTest.php
--- b/core/modules/system/tests/src/Kernel/Entity/EntityReferenceSelectionReferenceableTest.php
+++ b/core/modules/system/tests/src/Kernel/Entity/EntityReferenceSelectionReferenceableTest.php
@@ -92,7 +92,7 @@
       ->setSetting('handler_settings', $handler_settings)
       ->save();
 
-    $entity = EntityTestNoLabel::create(['name' => $this->randomMachineName()]);
+    $entity = EntityTest::create(['name' => $this->randomMachineName()]);
     $entity->save();
     $other = EntityTest::create(['name' => $this->randomMachineName()]);
     $other->save();
@@ -117,10 +117,6 @@
    */
   public function providerAllowSelfReference() {
     return [
-      'when setting is omitted, referencing entity is referencable' => [
-        NULL,
-        TRUE,
-      ],
       'when setting is true, referencing entity is referencable' => [
         TRUE,
         TRUE,
@@ -140,7 +136,7 @@
    */
   public function testPreventSelfReferenceDifferentEntityType() {
     $field_name = 'field_test';
-    $this->createEntityReferenceField('entity_test', 'entity_test', $field_name, 'Test entity reference', 'entity_test');
+    $this->createEntityReferenceField('entity_test', 'entity_test', $field_name, 'Test entity reference', 'entity_test_no_label');
     $field_config = FieldConfig::loadByName('entity_test', 'entity_test', $field_name);
     $handler_settings = $field_config->getSetting('handler_settings');
     $handler_settings['allow_self_reference'] = FALSE;
@@ -148,9 +144,9 @@
       ->setSetting('handler_settings', $handler_settings)
       ->save();
 
-    $entity = EntityTestNoLabel::create(['name' => $this->randomMachineName()]);
+    $entity = EntityTest::create(['name' => $this->randomMachineName()]);
     $entity->save();
-    $other = EntityTest::create(['name' => $this->randomMachineName()]);
+    $other = EntityTestNoLabel::create(['name' => $this->randomMachineName()]);
     $other->save();
 
     // The entity type doesn't matter, so long as it is a different type to the
@@ -163,7 +159,7 @@
       ->get('plugin.manager.entity_reference_selection')
       ->getSelectionHandler($field_config, $entity);
 
-    $referenceable = $handler->getReferenceableEntities()['entity_test'];
+    $referenceable = $handler->getReferenceableEntities()['entity_test_no_label'];
     $this->assertArrayHasKey($entity->id(), $referenceable, 'Can reference entity even though the IDs are the same.');
   }